McIntosh amp help

[deepinton]

Level matched, double blind, change only one variable at a time.

 

[ski2xblack]

You think something else in the amp could be causing the input selection issue, or something else in the amp could be causing my perceived "low power" issue? Any idea where else I should look besides the selector switch?

Well, yeah, it's possible that something else in there is faulty. You did describe them as "well used" after all. I hope not, for your sake, but anything is possible. Cleaning the input switch with deoxit is a good idea, low hanging fruit and all that. But unless you really know what you're doing, I suggest you don't start probing around in there.

The input sensitivity difference is responsible for less percieved power. You said the Classe needs only 1.37v (seems more like an unbalanced input sensitivity?? Does the classe specify balanced input sensitivity?) to hit rated power, while the Mac needs 4.2v on it's balanced inputs, 2.1v unbalanced. (I am not sure if Gene bench tested your model Marantz, but it should be more than capable of swinging those voltages from it's preouts. You will have to crank the volume higher in order for it to reach those higher voltages.)

 

[highfigh]

Is the labeling for the switch for 'Balanced or Unbalanced mode?

 

[PENG]

So the expected function of that switch is to specifically cutoff the XLR input if it is set to unbalanced? Would a malfunctioning switch attenuate the input level? My power output suspicion is driven by ear - my old Classe CAV 180 (5x180 watts) sounds louder at the same pre-pro output level, all else in the chain being the same.

Maybe some amps would do what you expected, but some amps just don't, example: Parasound, Marantz, Bryston etc., based on the models that I had used in the past. So, the Mc501 probably would behave like those amps, that is, the input selector are not really a physical selector switch, but one that was designed to maximize SNR, so if you use balanced output from a preamp, and connect to the Mc's XLR input and you flip the switch to unbalanced, you would likely get louder sound at the same preamp volume setting.

In that case, if you want to keep the volume low, but still get loud enough sound for you then do that, but it would make no sense because you will get poorer signal to noise ratio. My suggestion is, just use that selector switch according to the instructions, and always use balanced except of course when you have use preamps that have only unbalanced outputs.

Note: If you read the manual, the Mc501 specified:

Sensitivity 2.1 Volts Unbalanced Input 4.2 Volts Balanced Input

That confirms what I thought, that if you use the switch properly, you will hear no difference in sound pressure level at any given preamp volume setting whether you connect your preamp RCA output to Mc501 RCA input, or preamp XLR output to Mc501 XLR input.

 

[highfigh]

Maybe some amps would do what you expected, but some amps just don't, example: Parasound, Marantz, Bryston etc., based on the models that I had used in the past. So, the Mc501 probably would behave like those amps, that is, the input selector are not really a physical selector switch, but one that was designed to maximize SNR, so if you use balanced output from a preamp, and connect to the Mc's XLR input and you flip the switch to unbalanced, you would likely get louder sound at the same preamp volume setting.

In that case, if you want to keep the volume low, but still get loud enough sound for you then do that, but it would make no sense because you will get poorer signal to noise ratio. My suggestion is, just use that selector switch according to the instructions, and always use balanced except of course when you have use preamps that have only unbalanced outputs.

Note: If you read the manual, the Mc501 specified:



That confirms what I thought, that if you use the switch properly, you will hear no difference in sound pressure level at any given preamp volume setting whether you connect your preamp RCA output to Mc501 RCA input, or preamp XLR output to Mc501 XLR input.

I installed a system with Krell AV preamp and 5 channel power amp- I had requested that the electrician wire the outlets across the wall behind the cabinet a specific way and they don't seem to hav done that because it hummed with XLR cables, quiet with unbalanced. I would really like consumer and prosumer equipment to be designed for function, not marketing.

 

[deepinton]

Is the labeling for the switch for 'Balanced or Unbalanced mode?

Yes, the switch label says "Balanced/Unbalanced"

 

[deepinton]

Maybe some amps would do what you expected, but some amps just don't, example: Parasound, Marantz, Bryston etc., based on the models that I had used in the past. So, the Mc501 probably would behave like those amps, that is, the input selector are not really a physical selector switch, but one that was designed to maximize SNR, so if you use balanced output from a preamp, and connect to the Mc's XLR input and you flip the switch to unbalanced, you would likely get louder sound at the same preamp volume setting.

Everything (AKA Gemini) seems to point to this -
"A properly functioning INPUT MODE selector switch on the McIntosh MC 501 would completely cut off the XLR balanced input when you move the switch to the "unbalanced" position." Since I can hear XLR input with switch on "unbalanced" position and with nothing connected to the unbalanced input, step 1 is to start with the switch. I'll keep the channel posted in case folks are curious.

 

[PENG]

Everything (AKA Gemini) seems to point to this -
"A properly functioning INPUT MODE selector switch on the McIntosh MC 501 would completely cut off the XLR balanced input when you move the switch to the "unbalanced" position." Since I can hear XLR input with switch on "unbalanced" position and with nothing connected to the unbalanced input, step 1 is to start with the switch. I'll keep the channel posted in case folks are curious.

I did not and would not rule out there's an issue with the switch, but just saying you likely would not hear a difference in spl at any given volume setting of your preamp, because Mc501 keeps the gain the same whichever input you selected. As far as whether the switch has become defective, if your "Gemini" is a reliable source, then it would indicate that the switch may an issue, I have no idea what Gemini is, is it a forum? Have you tried to contact McIntosh and ask them directly?

The manual of a newer Mc amp has more info on such unbalanced/balanced "mode" selector switch:

From owner's manual of the MC2.1kW:

MODE – Set the MODE switch to the appropriate
position depending on the type of input being used.
Only one type of input (UNBAL or BAL) should
be connected to INPUT 1.

The fact that it says only one should be connected, would indicate the "mode" or input selector would/may not "cut off" the input that is not selected. If the switch does function as a typical and physical "input selector" switch that would only connect to one, the selected input signal, then they should allow connecting both input types as it would have been a welcome convenience feature.

Without knowing what the "Gemini" is, I remain skeptical, that is, the switch/selector may or may not disconnect the input that is not selected. The fact is, some amps do and some amps don't, so I would just put it a 50/50 probability.

Please do keep us posted, as I am curious, probably others are too.

 

[highfigh]

The easiest way to unbalance an XLR input is to connect Pins 1 and 3- if McIntosh chose to use a more complex method, I might guess that these have a small transformer for it. Just don't expect it to liik like a typical transformer- audio transformers for level and impedance matching are often small cylinders.

 

[PENG]

The easiest way to unbalance an XLR input is to connect Pins 1 and 3- if McIntosh chose to use a more complex method, I might guess that these have a small transformer for it. Just don't expect it to liik like a typical transformer- audio transformers for level and impedance matching are often small cylinders.

Found the answer, from an available schematics for the input connections, that is part of a 18 page downloadable file:

Download McIntosh MC501 Service Manual | ManualsLib

That shows what the OP found is normal, and the diagram above shows XLR input would work even in the selector is in the unbalance position. So, again, I wonder who/what that "Gemini" remark is about, doesn't really matter but I am being curious about it that's all.

If unbalanced RCA input is used, obviously one should not connect the XLR input in this sort of arrangement.

 

[deepinton]

Found the answer, from an available schematics for the input connections, that is part of a 18 page downloadable file:

Download McIntosh MC501 Service Manual | ManualsLib



View attachment 74576

That shows what the OP found is normal, and the diagram above shows XLR input would work even in the selector is in the unbalance position. So, again, I wonder who/what that "Gemini" remark is about, doesn't really matter but I am being curious about it that's all.

If unbalanced RCA input is used, obviously one should not connect the XLR input in this sort of arrangement.

Thanks a bunch for taking the trouble to dig through. I found the service manual and saw those circuit schematics as well, and managed to pull the board out to test with a multimeter and can confirm that your analysis is correct. That mode switch is not a hard switch. And Gemini is an AI agent that is supposed to research and aggregate available literature/online materials to answer questions. Just shows we humans still have an edge

 

[PENG]

I might guess that these have a small transformer for it. Just don't expect it to liik like a typical transformer- audio transformers for level and impedance matching are often small cylinders.

Not likely, more likely they would invert the signal to form the other half, so you would still get the

Thanks a bunch for taking the trouble to dig through. I found the service manual and saw those circuit schematics as well, and managed to pull the board out to test with a multimeter and can confirm that your analysis is correct. That mode switch is not a hard switch. And Gemini is an AI agent that is supposed to research and aggregate available literature/online materials to answer questions. Just shows we humans still have an edge

Thank you for the feedback on the Gemini thing. AI of this sort are very useful as you get quick and generally reliable information, but very often not precise enough for us Audioholics consumption lol...

 

[deepinton]

Not likely, more likely they would invert the signal to form the other half, so you would still get the


Thank you for the feedback on the Gemini thing. AI of this sort are very useful as you get quick and generally reliable information, but very often not precise enough for us Audioholics consumption lol...

Just out of curiosity, I pushed back on Gemini and asked it to review the circuit diagrams. It generated a long report that basically now agrees with your analysis. I don't know how to attach a PDF, but here's a link to the report - https://docs.google.com/document/d/1kNHzkPMGuTGo1BCY9-kSU6UW7U1eu54jhLNaRtPo8rM/edit?usp=drivesdk

Executive Summary
This report provides a detailed technical analysis of the McIntosh MC501 monoblock power amplifier's input selector architecture, focusing on the functionality of the switch and its associated circuitry. The core finding is that the MC501’s input selector is not a simple mechanical switch but a sophisticated, logic-controlled, relay-based system. This design allows it to seamlessly integrate both balanced (XLR) and unbalanced (RCA) input signals into the amplifier’s proprietary Quad-Differential circuitry. The analysis confirms that the input stage features two distinct signal paths with precisely matched electrical characteristics, a design choice that is crucial for maintaining the amplifier's low noise and distortion performance. The observed 6 dB difference in input sensitivity between the balanced and unbalanced inputs (4.2V vs. 2.1V) is a direct consequence of this design and serves as a key diagnostic indicator.
The report also examines common failure modes, such as oxidized relay contacts and control circuit malfunctions, which can lead to symptoms like intermittent audio or a completely non-functional switch. Diagnostic procedures using a Digital Multimeter (DMM) and an oscilloscope are detailed, along with recommendations for safely troubleshooting the unit. The report concludes with a synthesis of these findings, emphasizing that the input stage is an integral part of the amplifier's overall design, reflecting McIntosh's commitment to robust protection and long-term reliability.

 

[eljr]

Well exotic cables would indeed be a useless effort generally....just more audiophoolya. Your expectations could be the issue more than anything else....

Yes, I believe this is the ansewer. We only listen at a few watts so having 500 should not change anything except the fun in owning Mac amps so big and heavy to stare at.

I just sold my 501"s
They were great. Just sit back and enjoy.

 

[deepinton]

Well, I hope to get my units going again, do a double blind comparison with Hypex and resolve for myself the class D vs AB debate. If I can't tell the difference, or the Hypex comes out ahead, I am selling all these back breakers and going all in on class D.

 

[Golfx]

Well, I hope to get my units going again, do a double blind comparison with Hypex and resolve for myself the class D vs AB debate. If I can't tell the difference, or the Hypex comes out ahead, I am selling all these back breakers and going all in on class D.

Math is on the side of Class D

 

[deepinton]

Math is on the side of Class D

I'd agree from a math/objective perspective, but nothing beats a double blind test to resolve mental/subjective conflicts. Id also add that class D can only get better over time whereas AB appears to have peaked a while back.

 

[Golfx]

I think Denon/Marantz wisely continue to use Class AB because they are a known reliable entity to their manufacture.

 

[deepinton]

I think Denon/Marantz wisely continue to use Class AB because they are a known reliable entity to their manufacture.

Incumbents typically continue to do what they do/know best. Disruptors have to innovate and come up with technology that not only beats, but leapfrogs the incumbents. Again, I am not taking sides here as I have yet to make up my mind.